Figure 1 (below). The left micrograph shows an artery (A) and a vein (V), side by side, as they often appear in the body. The artery has a thicker wall, that helps it to maintain a circular cross section. The thinner walled vein is partially collapsed, and thus has an irregular profile.

The right micrograph is of the same vessels at a higher magnification. The solid bar shows the location of the arterial wall. The dashed bar indicated the wall of the vein.